The most recent standard solar cell holder for space flight and high-altitude experimentation of solar cells was developed by Near Space Characterization of Advanced Photovoltaics (NSCAP), which is a partnership between Air Force Research Laboratory (AFRL), Naval Research Laboratory (NRL), and National Aeronautics and Space Administration (NASA). The NSCAP solar cell holder is a 2-piece design that include a separate printed circuit board (PCB) connected to a 0.20-inch thick aluminum plate.
Although in use today, the NSCAP holder is not without limitations. Integration of a solar cell sample into this holder is a non-trivial process that includes: (i) a space-qualified silicone adhesive used to mount the cell to the aluminum plate; (ii) electrical connections that are made to a separate PCB, which fits on top of the aluminum plate: (iii) an analog temperature sensor that is mounted in a thermally conductive potting compound to the backside of the aluminum plate and is also electrically connected to the PCB; and (iv) the solar cell and temperature leads are connected to a costly, custom electrical connector that interfaces with an external source measurement unit (SMU).
A typical external SMU is a Keithley 2425, which is a bulky and heavy laboratory-grade instrument. The bulky footprint of this combined high altitude solar cell instrumentation package limits solar cell measurement opportunities to an expensive, high altitude balloon or aircraft programs such as government sanctioned balloon flights, NASA Learjet or ER-2 aircraft. Although the Learjet or ER-2 aircraft may fly on an approximate annual basis, only a small area is set aside for hosted experiments on these aircraft. Therefore, only a small number of samples can be flown per flight, further adding to the cost. A high-altitude balloon using the NSCAP package may fly more solar cell samples than the high-altitude aircraft, but the total weight does not decrease, again limiting the number of flights and driving up cost. A common cost estimate for flying a NSCAP balloon is approximately $1M to 2M.
Balloon-flown solar cell calibration standards do not exist following the last JPL campaign in 2005, which is direct evidence for the lack of flights. A similar lull in activity applies for the international institution Centre National D'Etudes Spatiales (CNES), which also flies balloons for solar cell calibration. CNES plans to fly a (heavy) high altitude balloon that includes custom current-voltage electronics and temperature control. CNES claims that this balloon will fly more than 50 solar cells. However, the cost of the campaign will be the equivalent to approximately $1M US dollars, and it is unknown if it will fly again after this flight.
As mentioned above, the Learjet and ER-2 fly approximately once a year, and oftentimes, only one of the aircraft will fly. The last ER-2 flight was May 2016 and the Learjet has not flown in almost two years. Two companies—BlackSky Aerospace Systems, LLC and Angstrom Designs Inc.—have developed inexpensive balloon platforms with flight operating cost approximately a tenth of the cost of operating the JPL or CNES balloon. These two companies, however, have not yet proven reaching altitudes greater than 100,000 feet and have not demonstrated the ability to perform accurate measurements.
The NSCAP holder also presents technical limitations that remain to be quantified. A thermal gradient between the solar cell, silicon adhesive, aluminum plate, and potting compound of the temperature sensor presents a challenge on precise measurement of the solar cell temperature. The NSCAP partnership has not demonstrated whether the temperature sensor configuration enables accurate measurement of the solar cell temperature. Furthermore, the temperature sensor outputs an analog current that must be converted to a digital voltage value. The conversion of values from analog to digital differs for different external SMUs and introduces variable amounts of error.
Thus, an alternative solar cell measurement unit may be beneficial.